1. Field of the Invention
The present invention relates to a reflector. More specifically, the present invention relates to a reflector used in a reflective type liquid crystal display device and to a reflective type liquid crystal display device having such a reflector incorporated therein.
2. Description of the Related Art
In recent years, while applications of liquid crystal display devices to apparatus such as personal computers, television sets, word processors, video cameras and the like have been promoted, demands for improved functions for such apparatus, including reduction in size, power consumption, and cost and the like, have been increasing. As a display capable of satisfying these demands, a reflective type liquid crystal display device which displays images by reflecting externally incident ambient light without using a backlight has been developed.
Since such a reflective type liquid crystal display device does not use any backlight, it is important to illuminate a screen of the display device by effectively utilizing ambient light. Accordingly, a reflector incorporated in the reflective type liquid crystal display device plays an important role. For obtaining a reflector having optimum reflection characteristics, various studies have been made.
For example, Japanese Laid-Open Publication No. 9-292504 discloses a technique of forming irregularities randomly with high density on a surface of the reflector. An object of the technique is to prevent an interference of light caused by a repetitive pattern of irregularities by increasing randomness of the irregularities, so as to prevent reflected light from being tinged with colors. Another object of the technique is to reduce a flat area by increasing the density of the irregularities, so as to diminish the amount of components of regular reflection.
Japanese Laid-Open Publication No. 57-102680 discloses a technique of limiting an average value of inclination angles of irregularities in order to collect scattering light within a certain range of area, and regulating a distribution of inclination angles of irregularities so as to have a peak therein in order to increase the intensity of scattering light when observed in a specific direction.
However, in the above described techniques, the distribution of inclination angles of irregularities for obtaining an ideal reflector is not theoretically supported. Since the distribution of inclination angles of irregularities is not controlled based on any theory or is controlled in a wrong way, the resultant fabricated reflector does not provide ideal optical characteristics.
In the technique of Japanese Laid-Open Publication No. 9-292504, the number of inclination angles of 0xc2x0 to 2xc2x0 is set to be larger than that of inclination angles of 2xc2x0 to 4xc2x0, so that the amount of components of regular reflection becomes larger; as a result, display quality decreases. In addition, since a light source image is reflected on the screen of the display in a portion of regular reflection, an observer keeps the light source image out of his field of view. Thus, the portion of regular reflection does not contribute to the brightness of the display.
On the other hand, in the technique disclosed in Japanese Laid-Open Publication No. 57-102680, while the brightness of the display when observed in a specific direction is improved, uniformity of display over the whole surface of a panel is not considered at all. Therefore, a significant difference in brightness is generated between opposite corners of the substrate, thereby decreasing the display quality. In addition, since there are limitations to the direction of a light source and the observation angle, the resultant reflector is inconvenient for use thereof.
The present invention is directed to solving the above described problems of the conventional techniques. An another object of the present invention is to provide a reflector having ideal reflection characteristics and a reflective type liquid crystal display device having excellent display quality in which the reflector is incorporated.
According to one aspect of the present invention, a reflector having irregularities on a surface thereof is provided. In the reflector of the present invention, a distribution of inclination angles of the surface is regulated such that a number of an inclination angle of the surface increases along with an increase of the inclination angle at least in the range from 0xc2x0 to 4xc2x0.
In one embodiment, the distribution of inclination angles of the surface is regulated such that the number of an inclination angle of the surface increases along with an increase of the inclination angle at least in the range from 0xc2x0 to 10xc2x0.
In another embodiment, a ratio of a reflectance in the direction of 10xc2x0 from the direction of regular reflection to a reflectance in the direction of 30xc2x0 from the direction of regular reflection is 10 or less.
According to another aspect of the present invention, a reflective type liquid crystal display device including a liquid crystal layer, a substrate and the reflector of the present invention is provided. In the reflective type liquid crystal display device of the present invention, the liquid crystal layer is sandwiched between the substrate and the reflector.
The inventors of the present invention have studied to obtain ideal reflection characteristics and developed an idea regarding a distribution of inclination angles of irregularities formed on a surface of a reflector. The present invention has been accomplished based on this theory.
Hereinafter, the function of the present invention will be described.
By setting a distribution of inclination angles of the irregularities formed on the reflector such that the number of inclination angles increases along with the increase of the inclination angle, the amount of scattered and reflected light beams is proportional to the area of a region which the light beams reach, as shown in FIG. 1 which will be later described. As a result, a substantially uniform density of light beams is obtained over the whole field of view.
However, in an actual reflective type liquid crystal display device, brightness is reduced to about ⅙ compared with the case where the reflector is solely used. Therefore, in order to increase the brightness, it is an optimum way to limit the direction in which light beams are scattered and collect the scattered light beams. In this case, in order to prevent a light source image from being reflected on the screen to increase the display quality, it is necessary to decrease the intensity of the scattered light beams in a direction of about 10xc2x0 from the direction of regular reflection.
Accordingly, in the present invention, the number of an inclination angle increases along with the increase of the inclination angle at least in the range from 0xc2x0 to 4xc2x0. Thereby, the amount of components of regular reflection is sufficiently reduced to make uniform the intensity of the scattering light beam in the range of 10xc2x0 from the direction of regular reflection, and to prevent a light source image from being reflected on the screen. Furthermore, since the number of inclination angles of 0xc2x0 to 2xc2x0 is lower than that of inclination angles of 2xc2x0 to 4xc2x0, the amount of components of regular reflection is further reduced in comparison with the conventional reflector. In this case, since a light source image is reflected on the screen of the display in a portion of regular reflection, an observer keeps a light source image out of his field of view. Therefore, actually the portion of regular reflection does not contribute to the brightness of the display. Thus, even when the amount of components of regular reflection is small, the display does not become darker. Additionally, by sufficiently diminishing the amount of components of regular reflection, the ratio of the brightness in the direction of regular reflection to the brightness in a direction inclined from regular reflection can be reduced. As illustrated in examples 1 and 2, which will be later described, a substantially uniform intensity of the reflected and scattered light beams is obtained in a region which the scattered light beams reach (e.g., a region of an angle of 30-45xc2x0 from the direction of regular reflection).
By regulating the distribution of inclination angles of irregularities such that the number of inclination angles increases along with the increase of the inclination angle at least in the range from 0xc2x0 to 10xc2x0, the amount of scattered and reflected light beams can be proportional to the area of a region which the scattered and reflected light beams reach in the range of the scattering angle of 30xc2x0 or less, which is required for an actual liquid crystal display device, as shown in FIGS. 4 and 5 which will be later described. Accordingly, substantially uniform density of light beams is obtained over the whole field of view. Thus, a display which is excellent in uniformity over the whole screen of the display device can be obtained.
The reflective type liquid crystal display device of the present invention is provided with a reflector of the present invention having excellent reflection and scattering characteristics. Therefore, a display which is excellent in brightness and uniformity can be obtained.
Thus, the invention described herein makes possible the advantages of (1) providing a reflector having ideal reflection characteristics, and (2) providing a reflective type liquid crystal display device having excellent display quality, in which the reflector is incorporated.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.